1. Field of the Invention
The present invention relates to an IC (Integrated Circuit)-type voltage controlled oscillator, and more particularly relates to a voltage controlled oscillator capable of preventing contraction of the operating frequency range caused by fluctuations of the frequency characteristics of each circuit due to fluctuations of the manufacturing conditions and variations of the operating temperature and the power source voltage.
2. Background Art
Conventionally, a voltage controlled oscillator which controls the output oscillating frequency in response to the change of the input voltage, is used in a variety of circuits. In conventional voltage controlled oscillators, the output oscillating frequency increases or decreases by xcex94F (Hz) monotonously with an increment xcex94V of the input voltage.
When a source voltage of 5V is used for such conventional circuits, a desirable input voltage-output frequency characteristic (which is called xe2x80x9cthe frequency characteristicxe2x80x9d, hereinafter) is obtained by obtaining the output oscillating frequency by incrementing the input voltage into the variable frequency oscillator shown in FIG. 6, using an inverter shown in FIG. 5.
However, as the integrated circuits becomes finer and their operating speed increases, the source voltage turns to a lower voltage such as a voltage of 2V or lower. In such a case, the operation of the variable frequency oscillator is liable to be saturated, and the relationship between the input voltage and the oscillating frequency begins to lose a linearity as shown in FIG. 7.
Fluctuations in the manufacturing conditions and variations in the operating temperature and the source voltage cause the frequency characteristic of voltage controlled oscillators to fluctuate and the best frequency characteristic is obtained for products, in which the manufacturing and environmental conditions are optimal, and the worst frequency characteristic is encountered for products under most unfavorable manufacturing and environmental conditions. That is, the frequency characteristic for a product subjected to optimal conditions (called the best condition) differs from that of a product subject to the most unfavorable conditions (called worst condition) as shown in FIG. 7.
Recently, as shown above, the frequency characteristic of IC-type voltage controlled oscillators tends to fluctuate due to the very small scale manufacturing process of devices constituting such voltage controlled oscillators and the lowered source voltage; and a first problem arises in that the operating frequency range becomes narrower as a result of the above-described tendency.
In order to obtain the broad operating frequency range of the frequency characteristic shown in FIG. 7, it is necessary to increase the gain, xcex94F/xcex94V, i.e., the rate of change xcex94F of the oscillating frequency when the input voltage is incremented by xcex94V.
However, since the analog potential is input into the input terminal of the voltage controlled oscillator, a simple increase in the gain generates an increase in the jitter, which is fluctuation of the oscillating frequency of a signal obtained from the output terminal.
When considering the variation of the frequency characteristics obtained over the range of the most unfavorable conditions (the worst condition) to the optimal conditions (the best condition), if the gain is set to an optimum value when the conditions are most unfavorable, a second problem arises in that the gain determined under the optimal conditions will be excessively large, and the above-described jitter will increase.
It is therefore an object of the present invention to provide a voltage controlled oscillator capable of being operated over a wide frequency range and with reduced fluctuation of the gain due to the fluctuation of the frequency characteristic.
The first aspect of the present invention provides a voltage controlled oscillator, which controls the output frequency by changing the input voltage, comprising: a variable frequency oscillator, whose oscillating frequency is controlled by the input signal; and a control device for controlling, said variable frequency oscillator such that a change of said input voltage by xcex94V volts changes the output frequency by xcex94F Hz, when said input voltage is equal to or less than a predetermined threshold voltage; and for controlling said variable frequency oscillator such that the change of said input voltage by xcex94V volts changes the output frequency by more than xcex94F Hz, when said input voltage exceeds a predetermined threshold voltage.
According to the second aspect of the present invention, in the voltage controlled oscillator according to the first aspect, the controlling device comprises a first voltage/current converter in which an output current changes in response to the change of the input voltage and a second voltage/current converter in which the output current changes in response to the change of the input voltage, when the input voltage exceeds the predetermined threshold voltage; wherein the variable frequency oscillator is controlled by the currents from the first and the second voltage/current converters.
According to the third aspect of the present invention, in the voltage controlled oscillator according to the second aspect, the second voltage/current converter is a voltage/current controller having a variable threshold voltage.
According to the fourth aspect of the present invention, in the voltage controlled oscillator according to the second aspect, the second voltage/current converter is comprised of a plurality of voltage/current controllers, each having a different threshold voltage.
When it is assumed that a voltage of the input terminal for a desired oscillating frequency of FOUT is VIN, and when the variation of the gain due to the fluctuation of the frequency characteristic of the oscillator is considered, the input voltage for obtaining a desired oscillating frequency FOUT is larger than VIN under the least favorable or the worst condition, and the input voltage becomes smaller than VIN at the time of the optimal or the best condition.
This is because the gain is smaller when the condition is most unfavorable or worst, and the gain is larger when the conditions are best, when compared to the gain under the normal condition.
For example, when it is assumed that the input voltage is equal to the VIN, under the most unfavorable conditions, the gain is increased in the vicinity of the desired frequency FOUT by the second voltage/current converter. That is, the variation of the gain due to the fluctuation of the frequency characteristics can be reduced.
In addition, the reduction of the change of the gain makes it possible to prevent a contraction of the operating frequency range.
When the input voltage exceeds a predetermined threshold voltage, variation of the gain can be further reduced by the use of a plurality of the second voltage/current converters and the use of a plurality of threshold voltages can expand the operating frequency range.